<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        const array1 = [1, 2, 3, 4, 5];

        // 0 + 1 + 2 + 3 + 4
        const initialValue = 0;
        const sumWithInitial = array1.reduce(
            (accumulator, currentValue) => accumulator + currentValue,
            initialValue
        );

        console.log(sumWithInitial);
    </script>
</body>

</html>